Crimp can be defined as the non-linearity in fiber. For most of the man-made fibers employed in carpet manufacture, the crimp or bend in the fiber is induced by thermal/mechanical techniques. It can also be thought of as the difference between the non-linear (crimped) fiber and the straightened fiber (fiber extended). Crimp is important in carpet fibers because it provides bulk to the yarn by preventing two fibers from laying parallel to one another. As a result, the carpet tuft will have greater covering power, appear softer, and give better resistance to wear and abrasion, among other benefits.
Crimp is also useful in the processing of staple fibers. Crimp is particularly useful in the processing of high modulus fibers which are difficult to work with because of slipperiness.
The crimp for most carpet fibers is placed in the fibers using a stuffer box method is rarely uniform. The stuffer box technique produces fibers having a wavy, random zig-zag type crimp which is balanced. The randomness of the crimp which is obtained would seem to cause the fiber to appear to have a nonuniform crimp; however, if several fibers are viewed simultaneously, it can be seen that the crimp produced by this method is regularly irregular.
Crimp in the stuffer box is achieved by passing yarn(s) or tow(s) into a uniformly heated chamber which is at the temperature required to heat set the fibers in their crimped or non-linear configuration. As the yarns are forced into the chamber by feed rolls, it pushes against yarn which is already in the chamber, thereby causing the filaments to bend and buckle (crimp).
A weighted tube fitted into the top of the stuffer box governs the flow and quantity of yarn into the stuffer box. The frequency (crimps per inch) and the crimp amplitude of the fibers are controlled by regulating the speed of the feed rolls to that of the take up rolls as well as the weight of the tube. Crimp setting by these techniques can be done for single filaments or on multiple ends (tow) using the spunize technique. The crimps are generally characterized by numerous sharp bends.
In order to obtain crimp the fiber must undergo bending. During bending two types of stress modes are developed simultaneously. There is a tensile stress along the outer curvature of the fiber, while a compressive stress is acting on the inner portion of the bend.
A recent study of the affects of crimp on polyester fiber demonstrated that severe bending (example, V-type crimp) can result in extensive fiber damage. Even the rounded V-type fibers showed compression ridges on the underside of the crimp, while severely crimped fibers (V-type bends) had reduced mechanical properties due to compressive forces operating within the fiber. The result is a weaker fiber. It has also been found that such overcrimped fibers tend to take up dye preferentially on the underside of the bend and can be the cause for optical streaking in the resulting yarn. This comes about because the knee of the bend projects toward the surface of the yarn and hence are more visible to the eye. Since they will contain more dye the affect is a dark optically appearing streak. At the same time, because the dye tends to concentrate at these points, the remaining fiber tends to be deficient in dye and appear lighter.
It has been shown that crimp permanency after loading can differ between fiber producers and even among various types (e.g., bright and semidull) made by the same producer. Since some tension on fibers and yarn inevitably attends normal fiber processing it is to be expected that some loss in crimp definition will likely occur. This loss must be near identical from spindle to spindle, twister to twister, etc. otherwise the yarns will appear to be different since crimped fibers differ in appearance from uncrimped fibers as a result of the reduced-bulking factor. At the same time some fiber elongation is obtained during crimp removal which would tend to order the fiber microstructure. This could influence dyeing since a more ordered microstructure will take up dye differentially than fibers which have not undergone any elongation.
U.S. patent application Ser. No. 112,353 of McCullough et al, which is herein incorporated by reference, discloses one method for preparing novel non-linear carbonaceous fibers having physical characteristics resulting from heat treating stabilized polymeric fibers in the form of a fabric. There is described a process wherein the fabric is substantially irreversible heat set under conditions free of non-uniform stress and tension. In order to obtain fibers which are non-linear, it is necessary to deknit the fibers. Knitting and then deknitting the fabric to obtain non-linear carbonaceous fibers increases the cost in producing the fibers.
U.S. Pat. No. 2,245,874 to Robinson, discloses a method for forming curled fiber material by passing fibers over rollers under conditions to bend and stretch the fibers beyond elastic limits. Such a process cannot be used to produce the non-linear fibers of the invention.
U.S. Pat. No. 2,623,266 to Hemmi discloses the mechanical preparation of sinusoid or spiraloid crimped fibers. The fibers are heated and passed through a series of bars which impart a meander-like crimp. However, the fibers are formed in a crimped and stretched state.
It is desirable to provide a relatively inexpensive and simple method for producing non-linear fibers and tows.
It is further desirable to provide a method for producing non-linear fibers which does not require the prior formation of a fabric.
It is also desirable to prepare non-linear carbonaceous fibers without performing a knit-deknit operation.